This invention relates to microencapsulated particles. In a more specific aspect, this invention relates to microencapsulated particles that are useful in electroluminescent applications. This invention also relates to a process for the microencapsulation of these particles.
This invention will be described in detail with specific reference to the microencapsulation of phosphor particles. However, this invention will be understood as applicable to the microencapsulation of other substance particles, such as pharmaceuticals, organic solvents, organic oils, pigments, dyes, epoxy resins, inorganic salts, etc.
Microencapsulated particles are known in the prior art. Bayless et al. U.S. Pat. No. 3,674,704 (1972) discloses a process for manufacturing minute capsules, en masse, in a liquid manufacturing vehicle wherein the capsules contain water or aqueous solutions. This patent discloses a specific process for manufacturing minute capsules wherein the capsule wall material is poly (ethylene-vinyl acetate) that is hydrolyzed to a narrowly specified degree (38-50 percent hydrolyzed).
Bayless U.S. Pat. No. 4,107,071 (1978) discloses microcapsules having a capsule core material surrounded by a relatively impermeable, densified protective wall and also discloses a process of manufacturing such microcapsules.
General encapsulating processes which utilize a liquid-liquid phase separation to provide a capsule wall material which envelops the capsule core material to be encapsulated are disclosed in Miller et al. U.S. Pat. No. 3,155,590; Powell et al. U.S. Pat. No. 3,415,758; and Wagner et al. U.S. Pat. No. 3,748,277.
Other prior art references disclose the encapsulation of electroluminescent phosphors; for example, see Budd U.S. Pat. No. 5,968,698 (1999). Additionally, the prior art discloses the coating of luminescent powders with a coating which comprises silicon dioxide; see Opitz et al. U.S. Pat. No. 5,744,233(1998).
Phosphor particles are used in a variety of applications, such as flat panel displays and decorations, cathode ray tubes, fluorescent lighting fixtures, etc. Luminescence or light emission by phosphor particles may be stimulated by applications of heat (thermoluminescence), light (photoluminescence), high energy radiation (e.g., x-rays or e-beams) or electric fields (electroluminescence).
For various reasons, the prior art fails to provide microencapsulated particles having the desired properties of impermeability to moisture and extended release capabilities. Thus, there is a need in the industry for microencapsulated particles having significantly improved properties.
Briefly described, the present invention provides microencapsulated particles which have an increased resistance to the adverse effects of moisture and which are able to function over an extended period of time (i.e., extended release capabilities). The present invention also provides a process for the microencapsulation of these particles.
The above-described advantages of the microencapsulated particles of this invention are evident when compared to similar microencapsulated particles manufactured according to the prior art (that is, not manufactured according to the present invention).
As used in this application, the following terms have the indicated definitions:
xe2x80x9cImpermeability to moisturexe2x80x9dxe2x80x94the ability to prevent or substantially eliminate the intake of moisture and thereby avoid the adverse effects of moisture.
xe2x80x9cImprovedxe2x80x9dxe2x80x94as compared to microencapsulated particles that are disclosed in the prior art and are not microencapsulated according to the present invention.
As will be seen in greater detail below, the microencapsulated particles of this invention have other characteristics that are either equivalent to, or significantly improved over, the corresponding characteristics of the prior art microencapsulated particles.
Accordingly, an object of this invention is to provide microencapsulated particles.
Another object of this invention is to provide microencapsulated particles having improved impermeability to moisture.
Another object of this invention is to provide microencapsulated particles having extended release capabilities.
Still another object of this invention is to provide microencapsulated phosphor particles.
Still another object of this invention is to provide microencapsulated phosphor particles having improved impermeability to moisture.
Still another object of this invention is to provide microencapsulated phosphor particles having extended release capabilities.
Still another object of this invention is to provide a process for the microencapsulation of particles.
Still another object of this invention is to provide a process for the microencapsulation of particles to produce microencapsulated particles having improved impermeability to moisture.
Still another object of this invention is to provide a process for the microencapsulation of particles to produce microencapsulated particles having extended release capabilities.
Yet still another object of this invention is to provide a process for the microencapsulation of phosphor particles.
Yet still another object of this invention is to provide a process for the microencapsulation of phosphor particles to produce microencapsulated phosphor particles having improved impermeability to moisture.
Yet still another object of this invention is to provide a process for the microencapsulation of phosphor particles to produce microencapsulated phosphor particles having extended release capabilities.
These and other objects, features and advantages of this invention will become apparent from the following detailed description.
FIG. 1 is a chart showing the effect of exposure (measured in hours) on brightness (measured in foot lamberts) of microencapsulated electroluminescent phosphors and electroluminescent phosphors which have not been microencapsulated.
With reference to FIG. 1, when tested in a humidity cabinet for 1,000 hours, lamps containing phosphors that have been microencapsulated according to this invention showed only 34% degradation, which is 60% less degradation than shown by electroluminescent lamps containing phosphors that have not been encapsulated.
In addition, when electroluminescent lights containing phosphors that have been microencapsulated according to this invention and incandescent lighting were tested as runway lights, the electroluminescent lamps produced no halos or glare and could be seen almost 3 times farther away than the incandescent lighting. This same result was observed under artic conditions.
FIG. 2 is a graphical representation of the relation between capsule quality and percent hydrolysis as applied to poly (ethylene-vinyl acetate), partially hydrolyzed. For reasons not entirely understood, the change in quality with change of percent hydrolysis is quite pronounced and remarkable. At hydrolysis of less than about 38 percent, the separated phase prepared according to established liquid-liquid phase separation techniques is not adequately viscous to form useful capsule walls, and the walls which are formed are sticky and generally unmanageable in attempts to isolate the capsules. Capsules made using materials having less than 38 percent hydrolysis have a tendency to agglomerate during the microencapsulation process, because a lack of vinyl alcohol groups prevents adequate cross-linking across hydroxyl groups.
At hydrolysis of greater than about 55 percent, the separated phase is too viscous and exists as a semi-solid, stringy, precipitous phase. The change from xe2x80x9cgoodxe2x80x9d to xe2x80x9cno-goodxe2x80x9d is abrupt and appears to be complete within a few percent.
At hydrolysis between 38 and 43 percent, quality capsules can be prepared with the quality improving as 43 percent hydrolysis is approached.
Between 43 and 53 percent hydrolysis, the capsule quality is excellent for this invention, and the capsules are particularly suited for containing phosphors, polar liquids and other substance particles for extended periods of time.
From hydrolysis of 53 to 54 or 55 percent, capsule quality declines rapidly, and at a hydrolysis of about 56 percent, quality capsules can no longer be successfully manufactured.
As represented in FIG. 2, at hydrolysis from about 44 to about 46 percent, the capsule quality is at a maximum for the present invention. The exact capsule quality values for this range of hydrolysis has not been specifically determined but, as represented in FIG. 2, is significantly improved over hydrolysis below this range.